Incandescent light sources have been used for many years in lighting fixtures. An incandescent light source, such as an incandescent light bulb, includes a filament, which when heated emits light. The filament in the incandescent bulb typically emits a light with a color temperature of about 3000 Kelvin (K) at full brightness, which is considered a “white” color. As the incandescent light source is dimmed by decreasing current, the filament emits a light that shifts away from a cooler color temperature, e.g., “white”, toward a warmer color temperature, such as yellow, orange, and then red. The color temperature change of an incandescent light bulb generally follow the color change of a cooling black body, i.e., a black body locus. An example of a dimming color curve for an incandescent light source, such as a halogen light source, is shown in FIG. 1.
Light Emitting Diodes (LEDs) have begun to replace traditional incandescent light sources in lighting fixtures. LEDs are efficient and reliable, and are able to emit a bright white light. Blue LEDs with a phosphor coating are typically used to produce white light. However, unlike traditional incandescent bulbs, the color temperature of an LED does not significantly change when the LED is dimmed. For example, when dimmed, the white light from an LED does not appear red but instead can become even more bluish.
One way to simulate the dimming characteristic of an incandescent lamp with an LED light source is to optically mix white LEDs with amber (e.g., yellow/orange) LEDs, and to control their currents in such a manner that the mixed color light from the LED combination changes from white tight to a more amber-yellow-white color with dimming. Traditionally, LED systems performing mixing of colored LEDs use individual drivers to control each colored LED separately, or use a single driver designed to have two or more separate output channels, where each output channel is controlled individually within the driver. An example of such a circuit is described in U.S. Pat. No. 7,288,902 to Melanson.